In about 3 months or so, my family and another one are going to a farm.I am planning to build a massive cannon.I want to be adventurous in its design so maybe advanced combustion or hybrid.But having no expirience on these, it probably wont work as well as it should.I am also considering making just a plain pneumatic or combustion.But if i made a pneumatic i would make a fancy quick load mechanism.

Since you have no experience, I say build a huge combustion that you can run on metered propane. It's much more fun not having misfires.Also, put a union or something in so you can use burst discs. It will make it more powerful and louder, something I really should have done on mine, instead of the Banjo fittings.

Mechanically they work fine. But I just don't like how they work. I have an over/under and my barrel isn't supported. So putting the barrel back on is a bit of a pain. I could remedy it by installing some sort of support and having it slide, but I haven't got around to it.

I would say build a pneumatic, they have a lot more power for their size. I would have said hybrid but since you can't do that, definitely go with a pneumatic. But, it all depends on how much money you are willing to spend...

Using 3 or 4 inch unions. That would certainly be powerful. Maybe do something like Gort's quick change system if you have the capability. If you don't have an air compressor, I would recommend a massive combustion that can fire at supersonic speeds...